Evolution and emergence of novel human infections

Arinaminpathy, N.; McLean, A. R.

Previous work on evolving pathogens has studied the effect of host heterogeneity (Yates et al. 2006 ) and pathogen life history (André & Day 2005) on the probability of emergence, per introduction into the human population. Here we ask: what are the epidemiological signs that a pathogen is evolving to adapt for human transmission? For example, under what conditions would such a process be signalled by large outbreaks of infection? Conversely, some pathogens, while capable of infecting humans, may face biological barriers to human adaptation that preclude their ultimate establishment. Is it possible, from case reports, to distinguish such pathogens from those inherently capable of adaptation? Finally, how can case reports be monitored to detect when an ongoing outbreak is about to develop into a full-blown emergence? We approach these questions using simple mathematical models of within-host evolution and between-host transmission. This paper is organized as follows: following a brief discussion of the relationship between pathogen reproductive fitness and outbreak sizes, we present a simple mathematical model of evolution and transmission, and apply it to the questions posed above. We use examples from H5N1 influenza case report data to illustrate the results. Finally, we discuss some public health implications of this work.